When used in applications such as brake fluids, functional fluids are typically exposed to metal surfaces including copper, zinc, aluminum and brass and to rubber parts, often under extreme conditions such as elevated temperatures leading to problems such as corrosion, oxidation and sediment formation. Higher under-the-hood temperatures in modern cars and trucks, the advent of antilock braking systems, and the desire for longer intervals between service visits has created a demand for high performance functional fluids with better resistance to corrosion, sediment formation and degradation over long periods of use.
Functional fluids typically comprise a base fluid containing glycols, glycol ethers, esters including borate esters and phosphate esters, ethoxylated or propoxylated alcohols, hydrocarbons and the like and a package of additives specifically designed to provide protection against corrosion of various metals, sediment formation and degradation. A variety of compounds are well recognized in the art as being useful as antioxidants, corrosion inhibitors etc., in functional fluids including certain triazole compounds. For example, amino-substituted 1,2,4 triazoles are disclosed in British Patent No. 1,111,680 as being a useful additive in ester based lubricant compositions. While these amino-substituted triazoles, exhibit good corrosion resistance for some metals, the data in Table II indicates some deficiencies in connection with copper and sediment formation.
Benzotriazole and its derivatives, in particular tolyltriazole are also recognized for their copper corrosion inhibiting properties and have been used as corrosion inhibitors in brake fluids . However, as pointed out in Japanese Kokai No. 59-157,188,these compounds are deficient in inhibiting copper corrosion and the formation of sediment in brake fluids. The Japanese Kokai patent suggests that the deficiencies of benzotriazole and its derivatives can be overcome by combining the triazole with one or more thiadiazole compounds. This combination, while effective in reducing sediment formation and inhibiting copper corrosion has the disadvantage of introducing a sulfur containing compound into the brake fluid which may have a negative effect on the long term heat stability of the fluid.